Chroma-key is a technique, commonly used in the production of television programs, in which a foreground subject is placed in front of a backing of a specific color, the whole being viewed by a television camera, and electronic circuits are employed to substitute a different background image wherever this specific color appears, thus producing the effect that the foreground subject is in front of the background scene.
Present chroma-key equipments suffer from several defects. Firstly, to avoid unwanted insertion of the background image into the foreground subject, it is necessary to avoid the presence of a large range of colors, centered on the backing color, within the foreground subject. Secondly, where the edges of the foreground subject are not sharply defined, the color of the backing wall appear around the edges of the foreground subject, creating a spurious `color halo` effect. If the user of the equipment attempts to avoid this problem by adjusting the equipment to suppress the foreground image when only a small amplitude of the backing color is detected, the desired diffuse detail of the edges of the foreground subject (for example, hair) will be sharply cut off.
This latter effect becomes more troublesome when the foreground video signal supplied to the chroma-key equipment is a composite encoded color signal, such as those according to the NTSC or PAL standards, rather than the red, green and blue signals originated by the camera. In encoded signals the bandwidth of the chrominance information is restricted, consequently transitions to and from the color of the backing are not sharply defined even if the foreground subject itself has sharp edges. With present chroma-key equipments operating from encoded signals it is usually necessary to set the equipment such that the edges of the foreground image are cut off to an objectionable extent, if color halo is to be avoided.
A third defect encountered when using encoded signals is that, due to the restricted chrominance bandwidth, fine detail of the foreground subject extending over the backing area is not passed in the chrominance signal and hence does not appear in the chroma-keyed image. Again hair is an example; strands of hair which appear in the original foreground image, being passed by the luminance bandwidth, do not result in a change in the chrominance signal (which controls the changing between foreground and background signals in the chroma-key equipment) and thus do not cause the chroma-key equipment to change from the background signal to the foreground signal as would be desired.